CA2178213C - Incremental backup system - Google Patents

Incremental backup system Download PDF

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Publication number
CA2178213C
CA2178213C CA 2178213 CA2178213A CA2178213C CA 2178213 C CA2178213 C CA 2178213C CA 2178213 CA2178213 CA 2178213 CA 2178213 A CA2178213 A CA 2178213A CA 2178213 C CA2178213 C CA 2178213C
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Prior art keywords
file
modification
data
means
backup
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Expired - Fee Related
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CA 2178213
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French (fr)
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CA2178213A1 (en
Inventor
Peter Bryan Malcolm
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Cheyenne Advanced Technology Ltd
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Cheyenne Advanced Technology Ltd
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Priority to US16538293A priority Critical
Priority to US08/165,382 priority
Application filed by Cheyenne Advanced Technology Ltd filed Critical Cheyenne Advanced Technology Ltd
Priority to PCT/GB1994/002631 priority patent/WO1995016237A1/en
Publication of CA2178213A1 publication Critical patent/CA2178213A1/en
Application granted granted Critical
Publication of CA2178213C publication Critical patent/CA2178213C/en
Anticipated expiration legal-status Critical
Application status is Expired - Fee Related legal-status Critical

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    • GPHYSICS
    • G06COMPUTING; CALCULATING; COUNTING
    • G06FELECTRIC DIGITAL DATA PROCESSING
    • G06F11/00Error detection; Error correction; Monitoring
    • G06F11/07Responding to the occurrence of a fault, e.g. fault tolerance
    • G06F11/14Error detection or correction of the data by redundancy in operation
    • G06F11/1402Saving, restoring, recovering or retrying
    • G06F11/1446Point-in-time backing up or restoration of persistent data
    • G06F11/1448Management of the data involved in backup or backup restore
    • G06F11/1451Management of the data involved in backup or backup restore by selection of backup contents
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10STECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S707/00Data processing: database and file management or data structures
    • Y10S707/99951File or database maintenance
    • Y10S707/99952Coherency, e.g. same view to multiple users
    • Y10S707/99955Archiving or backup

Abstract

In a method and system for operating a computer system, in particular a microprocessor-based computer system, modified areas of one or more files on a hard disk are monitored (22), and an auxiliary database is constructed (24) containing modification data sets relating to the location of those modified areas. On backup, the database is interrogated (30) and just those areas of files which have been modified are backed up (32) as a modification file. In this way the sue of the backup may be reduced. Subsequently data may be restored by interrogating the auxiliary data to identify which file areas are in the modification file, and selectively overwriting the data on the hard disk with backed-up data read from the modification file. The method and system may be incorporated in a dedicated backup application and/or operating system, or may interact with a conventional file-by-file backup application.

Description

. W095116237 PCTIGB94/02631 _1_ WCREMENTAL BACfCUP SY51EM

This invention relates to a method of operating a computer system, and in particular to a method of providing backup copies of data-to guard against data becoming corrupted or lost. The invention is particularly concerned with the provision of backups for personal computers (PCs).

Using current computer systems, file backup can be a time consuming operation. As long as a file to be backed up is fairly small, such as a word-processing file for example, then there are no major little problems.

However, a large multi-user database may have large files, often being larger than 1 Gigabyte. Currently, a fast tape drive is capable of a sustained data transfer rate of about 20 Megabytes/minute. A 1 Gigabyte file therefore takes a minimum of 51.2 minutes to back up. If backup copies of the database are to be kept up-to-date, then a single change in this file means that the whole file must be backed up. Furthermore, whilst the backup is in progress, no other user can modify the file without rendering the backup useless. Even if data is backed up overnight, for example, typically the backup window will only be 12 hours long, which is sufficient for only 14 Gigabytes to be backed up at these transfer rates. This assumes that a tape drive having an autochanger for replacing full tapes is available.

Large-scale file servers have already reached the point where the length of the backup window is insufficient and/or the volume of data to be backed up is _ too great for a conventional backup system having only one tape drive. The current approach to this problem is to build faster tape drives. typically by streaming data to two or more tape drives in parallel. However, the technical limitations of tape drive mechanisms suggest that, at least in the near future, data rates exceeding 1 to 2 Megabytes/second and approaching those of hard disk W0 95116237 ~ ( l ~ 2 ~ ~ PCTIGB94102631 systems are unlikely to be attainable. We have appreciated that since technical developments in storage systems are producing a rate of increase in hard disk capacity which exceeds the rate of increase in tape drive data transfer rates, the problem of bacJcing up large quantities of data will not be solved by building faster tape drives.
SUZ~IARY OE THE INVENTION
The invention in its various aspects is defined in the independent claims. Advantageous features are set forth in the appendant claims.
A preferred embodiment of the invention is described in-detail below with reference to the drawings.
In this embodiment, modified areas of one or more files on IS a hard disk are monitored, and an auxiliary database is constructed containing modification data sets relating to the location of those modified areas. On backup, the database is interrogated and just those areas of files which have been modified-are backed up as a modification file. In this way the size of the backup may be reduced.
Preferably, the auxiliary database is optimised by analysing the data sets to establish_whether any of them relate to the same or similar areas of the same file.
If the database contains data sets relating to the same area then only the latest one is relevant and earlier ones can be discarded. If several data sets relate to partially overlapping or contiguous areas then they may be concatenated into a single data set covering the total area formerly covered by the several data sets. In this way, the size of-the data base may be reduced and the number of accesses to a particular file during backing up and restoring may be reduced.
The auxiliary database, once created, contains a record of all modified areas of files modified since the last database was created (probably since the last backup depending'whether incremental, differential or full backups are being performed?. The database may be interrogated to establish which areas of which files have been modified, and-this information may~then be used to ~

permit the backing up of onlymodified areas ofa file rather than the whole file.
In order to perform a backup using a conventional file-by-file backup application, a ' S modification file is preferably created which contains the contents of the database and the data from the areas of the files referred to in the database. This file may then be backed up using a conventional backup application program.
Alternatively, the modification file may not actually exist as a file saved on the storage means, but may instead be simulated in such a way that the backup application interacts with it as though it were a file which does exist. This technique can be used to save IS time and/or storage space on the storage means.

A first preferred technique for simulating the modification file is to create a file on the storage means that is the same size as the modification file will be when it contains the auxiliary database and the associated areas ofmodified files. The simulated file contains redundant information and is only provided as an entity with which the backup application interacts as though it were a file of that size. Read requests from the backup application are monitored and, when the application attempts to read the simulated file, the data requested by the application is generated in real time and passed to the application instead of data from the file on the storage means. In this way, time is saved by not needing to write the modification file to the storage means and subsequently reading it back to pass it to the backup application:

A second preferred technique is to monitor directory read requests from the backup.application and to add details of the simulated modification file to the information returned by the operating system in such a way that the directory appears to include the modification file although no such file exists. When the backup application attempts to read the file, data is substituted in the same way as in the first technique. This technique saves space on the storage means by not requiring a file to be created on the storage means at all.
The method may be incorporated in an independent program which interacts with a separate operating system and/or backup application or it may form an integral part of one or both of these.
The method preferably includes a restoration phase which may be implemented as an independent program or as an integral part of an operating system and/or a backup application. The restoration phase may comprise interrogating the database to identify which areas are in the modification file, reading the backed up modification file, and subsequently writing backed up data to appropriate areas of files according to the modification data sets backed up as part of or with the modification file.
Thus, according to an aspect of the invention, there is provided a method of operating a microprocessor-based computer system including a random access memory, a central processing unit, and storage means, the method comprising the step of: providing instructions stored in the memory to the central processing unit to cause said central processing unit to write data to at least one file stored in the storage means by generating a write request; extracting information for a modification data set from each said write request; and building a database containing at least one said modification data set; wherein the write request comprises said modification data set which identifies a region of said file modified by said write request; and wherein the modification data set includes identification means for identifying said file, and a position identifier - 4a -for said file corresponding to the quantity of data to be written to said file.
The invention provides, in a further aspect, a microprocessor-based computer system including a random access memory, a central processing unit, and storage means, the system comprising: means for providing instructions stored in said memory to said central processing unit to cause said central processing unit to write data to at least one file stored in the storage means by generating a write request; means for extracting information for a modification data set from each said write request; and means for building a database containing at least one said modification data set; wherein the write request comprises said modification data set which identifies a region of said file modified by said write request; and wherein the modification data set includes identification means for identifying said file, and a position identifier for said file corresponding to the quantity of data to be written to said file.
The invention also provides a method of operating a microprocessor-based computer system including a random access memory, a central processing unit, and a storage means, the method comprising the steps of: providing an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified by a write request, each modification data set including identification means for identifying a position identifier for the file corresponding to the quantity of data to be written to the file; providing instructions stored in the random access memory to the central processing unit to command the following operations; interrogating the auxiliary database to establish which regions of the file have been modified; and creating a backup copy by backing up - 4b -data stored in the storage means, using information from the auxiliary database to indicate which data to backup.
In accordance with a still further aspect of the invention, there is provided apparatus for operating a microprocessor-based computer system which includes a random access memory, a central processing unit, and a storage means, comprising: an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified by a write request, each modification data set including identification means for identifying a position identifier for the file, corresponding to the quantity of data to be written to the file; means for providing instructions, stored in the random access memory, to the central processing unit; means for interrogating the auxiliary database to establish which regions of the file have been modified; and means for creating a backup copy by backing up data stored in the storage means, using information from the auxiliary database to indicate which data to backup.
According to another aspect of the invention, there is provided a method of accessing data in a microprocessor-based computer system including a random access memory, a central processing unit, a main storage means, and a backup storage means, the method comprising the steps of: providing an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified, each modification data set including identification means for identifying a position identifier for the file corresponding to the quantity of data to be written to the file; providing a modification data file containing regions of the file identified by each modification data set; and providing instructions stored in the random access memory to the - 4c -central processing unit to command the following operations;
interrogating the auxiliary database to establish which regions of the file have been modified; and restoring data from the modification data file to the main storage means using information from the auxiliary database to indicate which data to restore.
The invention provides, in a further aspect, apparatus for accessing data in a microprocessor-based computer system including a random access memory, a central processing unit, a main storage means, and a backup storage means, comprising: an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified, each modification data set including identification means for identifying a position identifier for the file corresponding to the quantity of data to be written to the file; a modification data file containing regions of the file identified by each modification data set; means for providing instructions stored in the random access memory to the central processing unit; means for interrogating the auxiliary database to establish which regions of the file have been modified; and means for restoring data from the modification data file to the main storage means using information from the auxiliary database to indicate which data to restore.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example, with reference to the drawings, in which:
Figure 1 is a block diagram of a personal computer and a tape drive;
Figures 2A and 2B are diagrams illustrating a system memory map at the operating system level of a - 4d -conventional personal computer, and a computer modified to operate in accordance with the method of one aspect of the invention, respectively;
Figure 3 is a flowchart of a file write process in accordance with an aspect of the invention;
Figure 4 is a flowchart of a backup process in accordance with a first embodiment of the invention;
Figure 5 is a flowchart of a backup process in accordance with a second embodiment of the invention; and Figure 6 is a flowchart of a backup process in accordance with a third embodiment of the invention.

2i78~i3 ~ WO 95116237 PCT/GB94102631 DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
With reference to Figure 1 of the drawirigs, a ' personal computer- (PC) 10 typically comprises a central processing unit (CPU) 12, a random access memory (RAM) 14, and a storage device in the form of a hard disk drive 16.
In this example, for the purpose of providing backup storage means, a tape unit 18 is coupled to the computer 10.
In order to reduce the volume of data that is required to be backed up on such a system, in accordance with this invention the size of the smallest entity that can be backed up (currently a single file) is reduced.
Currently, all file-by-file backup applications rely on the computer operating system to record which IS areas of the file system have been modified, and hence which areas require backing up. In all present PC based and Local Area Network (LAN) based operating systems, information relating to modified files is provided using a so-called "archive" flag, associated with each file, which is set when the file is modified. Each operating system also provides means by which the flag may be cleared when the file has been backed up.
In a preferred embodiment of the invention, means are provided for identifying particular areas of a file that have been modified rather than a whole file.
In this way a backup may be performed which is more efficient in its usage of backup storage capacity and time.
In a preferred embodiment, the system does not form part of an operating system or backup application.
Once the system has been activated to record modifications to files or to a particular subset of files, calls to the operating system to write to files are intercepted. With reference to Figures 2A and 2B, whenever a program wishes to acces-s a file, it calls a standard routine which writes data to the disk. This routine, which in the case of the DOS operating system is known as Interrupt 21. hex (INT2lh), is an integral part of the operating system.
The action performed by the routine depends on the ' 40 parameters passed to the routine upon entry. This routine is shown in Figure 2A as INTZlh.forming part of the operating system in a system memory map, the INT2lh entry point being shown by an arrow. To carry out a preferred method in accordance with the invention, additional program code is-added at the operating system interface level as shown in Figure 2B: " In practice, in a DOS environment, this can be loaded into the computer as a device driver using the CONFIG.SYS file.
The added software has the effect that an instruction to write data is replaced or supplemented by an alternative set of instructions. A similar technique may be used to intercept an instruction to read data from the disk and to replace or--supplement this also with an alternative set of instructions.
With reference to the flowchart of Figure 3, when an application performs step 20 to write data to a file, this is intercepted at the operating system interface level and steps 22 and 24 are inserted before the file write requested by the application is performed in step 26.
In step 22, using the information passed to the operating system as part. of step 20, the file to be written to is identified and a modification data set comprising the starting point or offset and the length of write are extracted. This data set uniquely identifies an area or portion of a particular file. In step 24, this data set is stored in an auxiliary database.
Control is then passed back to the write routine so that the write operation requested by the application is performed in the normal way (step 26). This sequence is repeated for each write request received by the operating system. In this way a database is constructed which contains a record of the location of all modified areas of files (although not of the data actually written to the files) .
If the auxiliary database-is-suitably indexed, ~
step 24 may also include a consolidation-operation which ensures that the database does not contain redundant information. For example, there is little point in the database retaining a modification data set relating to an _7_ area of a file that has subsequently been overwritten.
Earlier data sets are therefore discarded. Similarly if several modificatioh data sets-relate to overlapping areas of a file, these may be concatenated into a single modification data set relating to the total area .

associated with the several data sets.
The several data sets may then be discarded. Furthermore, the data sets may be ordered so that they form a sequential list related, for example, to the order of storage of the files on the storage means.

Since only the location of modifications and not the modifications themselves are stored in the database, the impact on the performance of-the computer system is minimal.

Having established the auxiliary database, there are several ways of backing up the areas represented by it.

In a first embodiment, a dedicated backup application is written or modified to take account of the database created according to the flowchart of Figure 3.

The operation of this arrangement is shown in Figure 4.

In step 28, the backup process is begun. In step 30, the backup application interrogates or queries the auxiliary database to establish which areas of which files have changed. At this point, the backup application may filter out those files which it is not wished to backup according, for example, to whether a particular file is included in a list of directory structure which has previously been specified by the user, in a conventional manner.

In step 32, a file which has been identified as modified is opened, and the data in the modified area identified by the database is copied to the backup media 18. Steps 30 and 32 are repeated until all files recorded in the database have been considered by the backup application and the modified areas recorded on the backup media 18. In addition to the data from the files, a copy of the database is also recorded. by the backup appiicationso that the backed up data may be identified during restoration. The information from the database _g_ may be distributed throughout the backup or may be recorded separately at the beginning or end of the backup, for example. Since this is the only data that need be backed up to kaep an up-to-date backup copy, and since typical users only modify very small parts or areas of a ' file, it will be appreciated that this backup cogy will be significantly smallerthan abackupcomprising all the data in each file that has been modified.
Three alternative embodiments which do not require a dedicated backup application program are described below.
Conventional backup applications are arranged to backup a complete file. In the first of the three alternatives, a modification file is created and stored on the hard disk 16. This file contains the-database information and the data stored in the file areas referred to in the database. A conventional backup application is then caused to backup the modification file and in so doing, backs up sufficient information to enable modifications-made since the last backup to be restored.
The two further alternatives are based around a phantom or simulated file.
In a second alternative, before the backup application is caused to backup the modification file as described above, a simulated or phantom file ~.s created which has the same size as that of the modification file were it to be created. No useful data~is stored in this file and its purpose is to provide an entry in the directory structure which can be interrogated by the backup application. By causing the backup application to attempt to backup this phantom file, the application will generate a series of read requests which can be intercepted at the operating system interface level as described above.
With referenceto Figure 5, in step 34, each read request is intercepted to check whether an attempt to read from the phantom file is being made and if so, the requested data is redirected in real-time. The redirection operation is performed in step 36. In this step, the backup application is not fed data,from the PCTlGB94/02631 phantom file but is instead fed redirected data which corresponds to that that would be read from the position in the modification file which corresponds to the requested position in the phantom file; i.e. it is fed the database information and the modified file areas. In this way, although disk space is taken up by the phantom file (which contains redundant data) the time taken to create and subsequently read the modification file is saved.
l0 In the third alternative embodiment, no phantom file is created on the hard disk 16. With reference to Figure 6, instead of creating a phantom file to provide an entry in the directory structure, the directory read request generated by the backup application is intercepted (step 38) in the same manner as the file read and write requests. In step 4D, information relating to a non-existent phantom file is added to the directory list returned by the operating system to the backup application. By intercepting file open requests and file read requests at the operating system interface level, the remainder of the backup procedure proceeds as described above for the second alternative. In this way, the waste ofdisk space caused by the creation of a phantom file containing redundant data is avoided.
In order to restore the backed up data, a modification data file that has previously been backed up is read. The auxiliary database is thereby extracted to identify which areas of which files are contained in the modification file. The respective areas on the hard disk 16 may then all or selectively be overwritten with the backed up data.
It will be appreciated that the embodiments described above may form an integral part of a backup application and/or of an operating system.

Claims (26)

1. A method of operating a microprocessor-based computer system including a random access memory, a central processing unit, and storage means, the method comprising the step of:
providing instructions stored in the memory to the central processing unit to cause said central processing unit to write data to at least one file stored in the storage means by generating a write request;
extracting information for a modification data set from each said write request; and building a database containing at least one said modification data set;
wherein the write request comprises said modification data set which identifies a region of said file modified by said write request; and wherein the modification data set includes identification means for identifying said file, and a position identifier for said file corresponding to the quantity of data to be written to said file.
2. The method of claim 1, further comprising the step of analysing a plurality of said modification data sets in said database and for each group of modification data sets relating to overlapping or contiguous write requests to replace each said group with a single respective new modification data set.
3. The method of claim 1, further comprising the step of interrogating the database to establish which regions of which files have been modified.
4. The method of claim 1, further comprising the step of creating a backup copy by backing up data stored in said storage means using information from said database to indicate which data to backup.
5. The method of claim 4, wherein the step of creating said backup copy comprises the steps of:
creating a modification file containing said modification data set or sets from said database and the region of each file identified by each respective set, storing said modification file on said storage means; and instructing a backup application program to make a backup copy of said modification file.
6. The method of claim 5, comprising the steps of:
reading a modification data set from said modification file;
reading said respective region from said modification file; and writing said region to the file specified in said modification data set.
7. The method of claim 4, wherein the step of creating said backup copy comprises the steps of:
generating a phantom file;
monitoring read requests generated by the backup operation; and responding to each said read request which corresponds to said phantom file by redirecting the read operation in real time to data identified in said data set or sets in said database.
8. The method of claim 7, further comprising the step of monitoring directory read requests from the backup application and adding details of said modification file to the information returned by the operating system in response to each said directory read request.
9. The method of claim 7, further comprising the step of creating a file containing redundant data having a size corresponding to the expected size of said modification file.
10. The method of claim 1, wherein the method is performed as an integral part of the operation of a backup application program.
11. The method of claim 1, wherein the method is performed as an integral part of the operation of a computer operating system.
12. A microprocessor-based computer system including a random access memory, a central processing unit, and storage means, the system comprising:
means for providing instructions stored in said memory to said central processing unit to cause said central processing unit to write data to at least one file stored in the storage means by generating a write request;
means for extracting information for a modification data set from each said write request; and means for building a database containing at least one said modification data set;
wherein the write request comprises said modification data set which identifies a region of said file modified by said write request; and wherein the modification data set includes identification means for identifying said file, and a position identifier for said file corresponding to the quantity of data to be written to said file.
13. The system of claim 12, further comprising means for analysing a plurality of said modification data sets in said database and for each group of modification data sets relating to overlapping or contiguous write requests to replace each said group with a single respective new modification data set.
14. The system of claim 12, further comprising means for interrogating the database to establish which regions of which files have been modified.
15. The system of claim 12, further comprising means for creating a backup copy by backing up data stored in said storage means using information from said database to indicate which data to backup.
16. The system of claim 15, wherein the means for creating said backup copy comprises means for:
creating a modification file containing said modification data set or sets from said database and the region of each file identified by each respective set;
storing said modification file on said storage means; and instructing a backup application program to make a backup copy of said modification file.
17. The system of claim 16, comprising means for:
reading a modification data set from said modification file;
reading said respective region from said modification file; and writing said region to the file specified in said modification data set.
18. The system of claim 15, wherein said means for creating said backup copy comprises:
means for generating a phantom file;
means for monitoring read request generated by said backup operation; and means for responding to each said read request which corresponds to said phantom file by redirecting the read operation in real time to data identified in said data set or sets in said database.
19. The system of claim 18, further comprising means for monitoring directory read request from the backup application and adding details of said modification file to the information returned by the operating system in response to each said directory and request.
20. The system of claim 18, further comprising means for creating a file containing redundant data having a size corresponding to the expected size of said modification file.
21. The system of claim 12, wherein the system forms an integral part of the operation of a backup application program.
22. The system of claim 12, wherein the system forms an integral part of the operation of a computer operating system.
23. A method of operating a microprocessor-based computer system including a random access memory, a central processing unit, and a storage means, the method comprising the steps of:
providing an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified by a write request, each modification data set including identification means for identifying a position identifier for the file corresponding to the quantity of data to be written to the file;
providing instructions stored in the random access memory to the central processing unit to command the following operations;
interrogating the auxiliary database to establish which regions of the file have been modified; and creating a backup copy by backing up data stored in the storage means, using information from the auxiliary database to indicate which data to backup.
24. Apparatus for operating a microprocessor-based computer system which includes a random access memory, a central processing unit, and a storage means, comprising:

an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified by a write request, each modification data set including identification means for identifying a position identifier for the file, corresponding to the quantity of data to be written to the file;
means for providing instructions, stored in the random access memory, to the central processing unit;
means for interrogating the auxiliary database to establish which regions of the file have been modified; and means for creating a backup copy by backing up data stored in the storage means, using information from the auxiliary database to indicate which data to backup.
25. A method of accessing data in a microprocessor-based computer system including a random access memory, a central processing unit, a main storage means, and a backup storage means, the method comprising the steps of: providing an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified, each modification data set including identification means for identifying a position identifier for the file corresponding to the quantity of data to be written to the file;
providing a modification data file containing regions of the file identified by each modification data set; and providing instructions stored in the random access memory to the central processing unit to command the following operations;

interrogating the auxiliary database to establish which regions of the file have been modified; and restoring data from the modification data file to the main storage means using information from the auxiliary database to indicate which data to restore.
26. Apparatus for accessing data in a microprocessor-based computer system including a random access memory, a central processing unit, a main storage means, and a backup storage means, comprising:
an auxiliary database containing modification data sets which identify regions of a file on the storage means that have been modified, each modification data set including identification means for identifying a position identifier for the file corresponding to the quantity of data to be written to the file;
a modification data file containing regions of the file identified by each modification data set;
means for providing instructions stored in the random access memory to the central processing unit;
means for interrogating the auxiliary database to establish which regions of the file have been modified; and means for restoring data from the modification data file to the main storage means using information from the auxiliary database to indicate which data to restore.
CA 2178213 1993-12-10 1994-12-01 Incremental backup system Expired - Fee Related CA2178213C (en)

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US16538293A true 1993-12-10 1993-12-10
US08/165,382 1993-12-10
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ES2131298T3 (en) 1999-07-16
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DE69418482D1 (en) 1999-06-17
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